This invention relates to methods and apparatus for measuring an electrical property of earth formations intersected by a borehole, and more particularly for simultaneously measuring an electrical property at different lateral distances from the borehole with "guard electrode" type investigating devices.
It is known that if measurements of formation resistivity are made at several lateral distances from a well bore, characteristics of the formation for producing oil and gas can be determined. A deep measurement will give apparent formation resistivity, R.sub.a, a shallow measurement will give the invaded zone resisitivity, R.sub.i, while a very shallow measurement of the formation immediately adjacent the borehole will give flushed zone resistivity, R.sub.xo.
The resistivity values may be used to determine when invasion has taken place, thereby detecting zones that have some permeability. Among other information, these logs are useful in detecting movable hydrocarbons present in the formation intersected by the borehole.
Long spaced and short spaced electrode systems have been used to measure some of these parameters. Guard electrode systems whereby current has been focused into a thin sheet to flow at a desired depth in the formation have been developed.
An electrode array using a plurality of electrodes, and varying one of the survey or auxiliary currents to establish and maintain a desired potential distribution for focusing a survey current to a desired depth in a formation has also been developed. As disclosed in U.S. Pat. No. 3,798,533 to Schuster, measuring the potential difference between various pairs of the electrodes in such an electrode array the resistivity of the noninvaded zone and the invaded zone may be determined.
The simultaneous use of long guard and short guard electrode systems has been suggested as showing promise for making resistivity measurements at different radial distances from the borehole, but no method for implementing this suggestion has been disclosed.
The combination of electrode systems with focused induction coils for logging two depths from a borehole have been disclosed, and a dual induction coil system has also been developed.
The dual induction coil apparatus is most preferred for use in boreholes filled with fresh mud. An apparatus using dual guard electrode systems is most preferred in boreholes filled with high salinity mud, and where high resistivity formations are encountered; although its use should not be understood to be limited to those applications.
In the present apparatus disclosed herein, two guard electrode systems are used, one having long guard electrodes for focusing the formation current emitted from an alpha ring between the long guard electrodes deep into the formation to be measured, and one having short guard electrodes for focusing the formation current emitted from an alpha ring between the short guard electrodes to a shallow depth into the formation to be measured. Deep and shallow are, of course, relative terms. The deep guard electrodes may be designed to measure the uninvaded formation, and the shallow guard electrodes may be designed to measure the invaded zone of the formation.
A circuit is disclosed which uses a single oscillator which simultaneously controls both the deep formation current and the shallow formation current to have the same frequency.
The logging cable armor acts as the current return for one guard electrode system, while a cable electrode immediately above the apparatus acts as the current return for the other guard electrode system.
In one embodiment, reference voltages for both guard electrode systems are taken between the guard electrode systems and a cable electrode located between the cable armor acting as a current return and the mentioned cable electrode acting as a current return.
In another embodiment, the reference voltage for one guard electrode system is taken between the guard electrodes of that system and a cable electrode located between that portion of the logging cable acting as a current return and the mentioned current return cable electrode. The reference voltage for the other guard electrode system is taken between the guard electrodes of that other system and a cable electrode on the logging cable located between the guard electrode systems and the mentioned current return cable electrode.
It has been found that a pad mounted electrode measuring system may be run with the dual guard electrode apparatus to make a simultaneous log of the flushed zone immediately adjacent the borehole without interfering with logs being made of the other desired depths by the guard electrode systems .